The two broad objectives of this research project are: 1) determination of the electrophysiologic and anatomic basis for the abnormal surface ECG potentials; 2) determination of the electrophysiologic and anatomic bases of cardiac arrhythmias. The aim of Project #1 is more accurate prediction of the abnormal cardiac anatomy and electrophysiology from surface potential (ECG) recordings. 2) The basis of the abnormal surface ECG potentials will be determined by recording the ventricular activation sequence and defining the pathologic anatomy associated with these changes. 3) Both animal models and human subjects with heart disease will be studied. 4) Epicardial mapping in human subjects, as well as epicardial, endocardial, and intramural mapping in the animal models will be performed. Project #2 is concerned with unraveling the various mechanisms of each specific cardiac arrhythmia. The basic hypothesis is that most cardiac arrhythmias result from abnormalities in conduction, refractoriness, and a triggering ectopic impulse, plus interaction between these three basic variables. Activation sequence maps will be produced from multiple electrode recordings. The single extrastimulus method will be used to determine refractory periods from multiple close cardiac locations. Maps of the global activation and the refractory field will be produced by computer. By correlation of the activation sequence maps, the refractory field maps, the location and timing of premature beats, and the abnormal anatomy of atrium, ventricle, specialized conduction system, etc., we expect to determine a specific mechanism for each cardiac arrhythmia.